By U.S. Pat. No. 7,387,474, an indexable turning insert is previously known, which has a rhombic basic shape and includes two diametrically opposed cutting edges, which individually comprise two main edges, which converge toward a common secondary or nose edge, which forms a rounded transition between the two main edges, and which always generates the exposed surface in the workpiece irrespective of which one of the two main edges that is active and provides for the active chip removal. In order to enable good machining results in respect of dimensional accuracy and surface smoothness irrespective of the index position of the cutting insert, it is important that the nose edges of the cutting insert are situated at exactly equally large distances not only from the center of the cutting insert (which in this case is represented by a through hole for a fixing screw), but also from the under side of the cutting insert. Only in such a way it is ensured that the active nose edge always assumes one and the same spatial position in relation to the basic body of the tool in which the cutting insert is mounted. For this reason, it is necessary to precision grind not only the upper side of the cutting insert but also the clearance surfaces that delimit the cutting edges together with the upper side. In this connection, the requirements of dimensional accuracy are often within the range of ±0.025 mm.
The same stiff requirements of dimensional accuracy are also made on cutting inserts having only one cutting edge. Thus, it is required that the single cutting edge is located in an accurately defined spatial position in relation to the support points of the cutting insert (e.g. rear side support surfaces or a groove in a lower connecting surface) for the nose edge to assume an exact predetermined position in relation to the basic body of the tool in which the cutting insert is mounted.
All forms of grinding of cemented carbide inserts are costly. The rule is that the cost increases with increasing grinding surface and increasing grinding depth. Another factor than the cost factor of great importance to the user is the quality of the grinding. A user responsible for machining an expensive workpiece needs of course to have reliance in not only the basic body of the tool but above all the replaceable cutting inserts, which gradually are consumed. Therefore, if a cutting insert would be impaired by even the slightest defective grinding, e.g. in transitions between different surfaces on the cutting insert, the ability of the cutting insert to machine the workpiece in a dimensionally accurate way would diminish. In this connection, it should be pointed out that also utmost small grinding defects will be visible to the naked eye by the reflection of the light from the ground surfaces. If the user would suspect that a cutting insert is defect, there is a risk that he directly discards the same.
Already grinding of genuine cemented carbide inserts, i.e., cutting inserts exclusively made from cemented carbide, is accordingly a delicate task, in particular if the pressing/sintering and the grinding, respectively, of the cutting inserts are carried out by different actors in different works. The task becomes even more delicate if the cemented carbide inserts in addition are to be provided with particularly hard insets precisely in the area of the cutting edges. When turning materials particularly difficult to machine, such as hardened steel, cast iron and the like, it is accordingly desirable to supplement the cemented carbide with a particular inset of a material, e.g. cubic boron nitride (CBN), and to form the individual cutting edge in the inset, more precisely by grinding a chip surface as well as a connecting clearance surface. Although such insets usually are only of millimeter size, they make the grinding significantly more difficult, not only in respect of time-consumption and cost, but also in respect of the quality of the finish-ground surfaces, in particular in transitions between the individual inset and the surrounding cemented carbide.
An example of a cutting insert that makes use of a superhard inset, in which the cutting edge is shaped, is disclosed in U.S. Patent Application Publication No. 2004/0258944 A1. A disadvantage of this cutting insert is, however, that the manufacture requires grinding of large cemented carbide surfaces in addition to the relatively small surfaces on the inset that are required to delimit the cutting edge. Thus, not only the chip surface of the hard inset but also the adjacent cemented carbide surface included in the upper side of the cutting insert has to be ground. In an analogous way, the cemented carbide surfaces surrounding the clearance surfaces of the inset have to be ground all the way from the inset to the center of the cutting insert. This means that unnecessary energy and time are consumed to grind in the inset while ensuring neat transitions between the inset and the surrounding cemented carbide.
The present invention aims at obviating the above-mentioned problems and at providing an improved cemented carbide insert of the type that requires simultaneous precision grinding of surfaces on an extremely hard inset as well as surrounding surfaces of cemented carbide. Therefore, an object of the invention is to provide a cemented carbide insert, in particular a turning insert, e.g. of the type that is disclosed in U.S. Pat. No. 7,387,474, which can be made with an inset difficult to grind of an extraordinary hard nature, and which in spite of said inset can be precision ground in a simple and cost-effective way.